1. Field of the Invention
The present invention relates to a circuit analysis system and more particularly to a circuit analysis and design systems for use with Direct Digital Frequency Synthesizers.
2. Description of the Related Art
Direct Digital Frequency Synthesizers, commonly referred to as DDS or DDFS, have an important role in wide areas of digital electronics. Before the development of direct digital frequency synthesizers, high bandwidth frequency synthesis was achieved by the use of phase-locked loop (PLL) implementations. There are a number of advantages offered by DDS, over PLL-type implementations, that are very important in terms of performance improvement and superior capabilities. For example, DDS offers instantaneous settling time, fast and continuous-phase switching response, very fine (virtually limitless) frequency resolution, large bandwidth, low phase noise, and excellent spectral purity.
The importance of direct digital frequency synthesis has increased in the past decades due to rapid advancements in semiconductor technologies and the development of its applications such as in digital communications, electronic warfare and radar systems, hydrogen maser receivers, particle accelerators, test and measurement equipment, broadcasting, and medical equipment. The importance of DDS in practical applications is expected to continue growing since the need for higher performance circuits and products is inevitable in future generation products. For example, FIG. 1 depicts an application of DDS 20 in a quadrature amplitude modulator (QAM) transceiver block 22, which is used in an array of digital communications products. In this QAM transceiver application the DDS takes a frequency control signal 24, precisely indicating the required carrier frequency, and generates signals with the samples of sine and cosine functions 26 and 28 having the desired carrier frequency. The DDS outputs are used to modulate information-bearing signals I-30 and Q-32 following processing of information from an information source 34 processing of the information into I and Q components by a symbol generator 36 and following pulse shaping filters 38 and 40 and the interpolation filters 42 and 44 to the appropriate carrier frequency for transmission through the communication channel 46 at the multiplier 48. The pulse-shaping and interpolation filter parameters, along with high frequency precision and accuracy of DDS outputs, play a crucial role in the overall level of performance for the QAM modulator.
In some applications, the DDS is used to generate only the sine or only the cosine output. FIG. 2 illustrates an example where a DDS-50 is used to generate a high precision (12-bit) cosine signal 52 in the digital domain, having a specified frequency and phase offset as inputs 54 and 56. The integrated circuit (IC) provides an analog output signal-58 by using a 12-bit digital-to-analog converter (DAC-60). Depending on the configuration of the IC, the DDS output can be fed directly into the DAC (not shown), it can be passed through an “inverse sinc” filter or a digital multiplier before feeding the DAC (not shown), or it can go through both the inverse sinc filter-62 and the digital multiplier 64 (FIG. 2) before reaching the DAC input-66. The inverse sinc filter compensates for distortion introduced by the DAC. The digital multiplier controls according to input signal-68 the amplitude of the generated sinusoid. Therefore, the IC is capable of generating a very high precision analog cosine waveform having a specified frequency, phase offset, and amplitude. With such capabilities the user can generate a wide variety of waveforms with frequency modulation, phase modulation, amplitude modulation, or any combination of these three modulations such as the bandpass pulse-amplitude-modulated (PAM) signal.
In both of the above-illustrated QAM modulator and cosine generator examples, as with other conventional DDS uses, the DDS plays a central role in the functionality and capabilities of the entire system in which it operates; overall system performance highly depends on the level of performance offered by the DDS. Designing a high performance and low cost DDS using conventional technology is a challenging task, one that has been pursued for several decades. The characterization of DDS performance is also a very challenging task. Although some research has been done to suggest techniques and algorithms for DDS characterization, the number of publications addressing this problem seems exceeded by the number of publications discussing various DDS implementations. The complexity of DDS characterization and the challenge presented by the need for a characterization algorithm that is applicable to any DDS with an arbitrary set of design parameters are the reasons for the absence of such a method to date. The present invention provides an exemplary solution to such DDS characterization problems when applied to any DDS implementation.